wait_task_inactive(): don't consider task->nivcsw

If wait_task_inactive() returns success the task was deactivated.  In that
case schedule() always increments ->nvcsw which alone can be used as a
"generation counter".

If the next call returns the same number, we can be sure that the task was
unscheduled.  Otherwise, because we know that .on_rq == 0 again, ->nvcsw
should have been changed in between.

Q: perhaps it is better to do "ncsw = (p->nvcsw << 1) | 1" ?  This
decreases the possibility of "was it unscheduled" false positive when
->nvcsw == 0.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -1921,11 +1921,8 @@ unsigned long wait_task_inactive(struct task_struct *p, long match_state)
 		running = task_running(rq, p);
 		on_rq = p->se.on_rq;
 		ncsw = 0;
-		if (!match_state || p->state == match_state) {
-			ncsw = p->nivcsw + p->nvcsw;
-			if (unlikely(!ncsw))
-				ncsw = 1;
-		}
+		if (!match_state || p->state == match_state)
+			ncsw = p->nvcsw ?: 1;
 		task_rq_unlock(rq, &flags);
 
 		/*